1. Field of the Invention
The invention relates to an apparatus for the deposition of thin films in the presence of a radio frequency (RF) glow discharge or "plasma." The apparatus of the invention is particularly suited for deposition of silicon nitride films as a final passivation step in the production of integrated circuits. However, the apparatus may be used to generally form films where the reactant gases possess widely varying binding energies and/or do not lend themselves to premixing.
2. Description of the Prior Art
Various ways are known in the art for applying a layer or film of material upon a substrate or base material. Such methods include spin coating with subsequent solidification and/or crosslinking, evaporation, sputtering, plating, chemical vapor deposition, and plasma deposition by the use of a RF glow discharge. Each such technique has its inherent advantages and disadvantages with respect to certain desired results. In the case of RF glow discharge plasma desposition, gas reactants are broken down into ionized species in the presence of radio frequency (RF) excitation; e.g., 13.56 mHz. It is theoretically possible, once the reactant gases have been ionized, to recombine the elemental species into a desired form. Alternatively, a particular film might be formed by chemical vapor deposition (CVD). However, CVD must be performed at high temperatures in the order of 1000.degree. C. to 1600.degree. C. and above. Such high temperatures often damage substrate structures which include aluminum conductors as aluminum melts at less than 600.degree. C. Alternatively, sputtering and electron beam evaporation are conceivable methods for forming a film, but the properties of a sputtered film may not exhibit the adhesion, density, or resistance to etching as required, and sputtering and evaporation have generally not been effective for non-conductors. In the end result, every thin film deposition problem must be dealt with on the basis of the particular technologies desired to be employed and the results needed to be achieved.
Specifically, the application of silicon nitride to the manufacture and the production of integrated circuits is critical. Silicon nitride films are utilized as a final passivating or protecting layer for an integrated circuit. The silicon nitride performs the function of an impermeable boundary between the corrosive and damaging atmosphere and the delicate integrated circuit thereunder. The alternative method in lieu of silicon nitride passivation is the sealing of the integrated circuit within a ceramic package. While the use of the ceramic packaging technique is technically sufficient to hermetically seal the integrated circuit, it is prohibitively costly for high volume production. It has, thus, become desirable to hermetically seal integrated circuits by use of a thin silicon nitride layer.
There are several problems, however, with silicon nitride films as currently applied by the teachings of the prior art. Silicon nitride may be applied by chemical vapor deposition, but the reaction temperatures in excess of 1000.degree. C. are often devastating to the integrated circuits that the silicon nitride layer is designed to protect. In fact, some integrated circuits should not be exposed to any temperatures in excess of 100.degree. C. In addition silicon nitride is an excellent insulator, and the semiconductor industry would consider using silicon nitride as an interlayer dielectric for forming capacitors and other active elements of integrated circuits, but the quality of the presently available silicon nitride films as formed by RF glow discharge plasma techniques are insufficient for such an application. Insulating characteristics of a dielectric material are evaluated by measuring the "flat band voltage" of the film which measures the capacitance of a test sample as a function of voltage applied. Porous films of unpredictable quality with considerable amounts of trapped charges and with poor and variable dielectric constants result from the use of prior art plasma deposition systems making silicon nitride films produced therefrom unacceptable in terms of their flat band voltage characteristics. In addition, silicon nitride films as produced by current technologies vary widely in the internal stresses resulting during deposition, etch rates of the film in suitable etchants, adhesion characteristics, imperviousness to sodium diffusion, and index of refraction. If silicon nitride plasma deposited films could be obtained of sufficient quality at a low enough application temperature, such films could be used for diffusion masking of gallium arsenide, a particularly temperature sensitive substrate used in the manufacture of light emitting diodes. To date such films have not been commercially achieved by the use of plasma reactors of the prior art. In fact, films deposited by RF glow discharge plasma deposition of silicon nitride vary so much that they bear little relationship to "beta" crystaline Si.sub.3 N.sub.4 which is the only absolutely determinable standard for comparison.
In the art concerned with plasma deposition of silicon nitride, U.S. Pat. No. 3,757,733 to Reinberg discloses the use of a radial flow reactor for the plasma deposition of silicon nitride and other film materials. The radial flow reactor is a radially symmetrical chamber in which substrates to be coated are arranged on a circular fixture at ground potential in a plurality of concentric rows. Reactant gases are introduced into the chamber concentric with the sample mounting fixture from below said fixture and circulate under the action of a vacuum pump to the top face of the fixture where the samples reside. The uppermost face where the samples are fixtured has a central port connected to the vacuum pump through which reacting gases exit. In other words, a radial flow of reacting gases is created from the inlet of the chamber to the exit from the chamber. In addition, the fixture upon which the samples are rigidly held is at RF ground potential, and an upper plate of the vacuum chamber forms the driven or "hot" electrode of the RF plasma system. When the RF source is energized, a plasma is struck between the upper plate of the vacuum chamber and the circularly shaped mount upon which the substrates are fixtured. Heaters are included as part of the substrate fixturing arrangement on the underside of the fixture in order to heat the substrates during deposition to temperatures of approximately 200.degree. C. to 400.degree. C. With the exception of rotation of the mounting fixture to which the substrates are held wherein such rotation is effected during the deposition process and the direction of flow of the reactant gases within the reaction chamber, the radial flow reactor as taught by Reinberg is similar to the Plasma I Low Temperature Nitride Reactor manufactured by Applied Materials, Inc., Santa Clara, Calif.
The radial flow reactor as taught by Reinberg suffers from several problems inherent in its design. First, the flow of the reactant gases, while theoretically symmetrical in entering or exiting from the reactor chamber, is inherently nonuniform in terms of the mass density of reactant gases as a function of radius across the substrate mounting fixture. In other words, the mass density of reactant gases must be greater at the center of the circularly shaped substrate mounting fixture than at its periphery. This creates an inherent reaction nonuniformity as a function of the location of the substrate radially upon the substrate mounting fixture.
Second, while it is desirable to utilize semiconductor grade pure diatomic nitrogen (N.sub.2) as the nitrogen donor gas and silane (SiH.sub.4) as the silicon donor gas in order to form silicon nitride, the binding energy of diatomic nitrogen is approximately three times the binding energy of silane. That is, on a statistical basis, it will be much less probable that the reaction chamber as taught by Reinberg will contain a theoretical three silicon ions to four nitrogen ions if approximately equal atomic proportions of silane and nitrogen are utilized, because they will not dissociate in such a relationship. In fact, Reinberg suggests that a good nitride deposition coating can be achieved by a fairly high nitrogen to silane ratio particularly on the order of about 200 to 1. This attests empirically to the inefficency of the RT glow discharge plasma dissociation of the nitrogen with respect to the silane. In order to get around this problem, it is not uncommon with the Reinberg reactor (as well as that manufactured by Applied Materials, Inc.) to introduce ammonia (NH.sub.3) as one of the component reactant gases as a nitrogen donor, because the binding energy of nitrogen to hydrogen in ammonia is approximately equal to the binding energy of the silicon to hydrogen bond in silane. The problems inherent with the use of ammonia are many: ammonia is an inherently corrosive gas, extremely harsh on the components of the reactor system including valves, conduits, and vacuum pumps; ammonia is difficult to obtain in extremely pure grades thereby introducing contaminants into the system; ammonia brings with it the unwanted byproduct of hydrogen during its dissociation which must be disposed of lest it end up as an unwanted contaminant within the silicon nitride film.
A third deficiency of the Reinberg reactor is its horizontal disposition of the substrates within the reactor chamber. The plasma deposition of silicon nitride creates particulates which are contaminants with respect to the ultimate deposition of silicon nitride upon the substrates. In a horizontally disposed reactor with respect to gravity, particulates inevitably fall upon the substrates to be coated and are thereby included as part of the film. The critical properties of films with particulate contaminants included therein are extremely degraded.
Fourth, the substrates in the Reinberg reactor are fixtured to the ground potential electrode of the RF glow discharge. Surrounding both plates of the RF glow discharge, the ground plate as well as the excitation electrode, are extremely electron-rich areas. It has been found that substrates which are in close proximity or directly mounted to RF glow discharge electrodes tend to be surface damaged by the atomic species and electron bombardment inherently formed in the vicinity of said electrodes. Thus, the Reinberg reactor tends to damage the substrate circuits and the silicon nitride films so produced by the grounding of the substrates within the plasma glow discharge.
A fifth deficiency with Reinberg is that any nonparallelism between the substrate mounting fixture at ground potential and the upper driven electrode of the RF glow discharge will cause a nonsymmetry in the plasma so created. This can cause the plasma to "flicker" on and off in certain areas or have "hot" spots of extremely concentrated ions known in the art as "fireballs." This will cause a nonuniformity of coatings produced by such a reactor. Two attempts have been made to reduce the nonuniformity of coatings produced by such radial reactor. The first method increases the flow rates of the reactant gases in conjunction with an argon carrier gas to an extent such that the reactant gases are "brute forced" into the chamber in order to urge the plasma into a circularly uniform configuration. Second, as incorporated in the Applied Materials reactor, the substrate mounting fixture is rotated so as to attempt to eliminate coating nonuniformities on a sample-to-sample basis. An increase in the flow rate of the reactant gases is an extremely inefficient method to improve deposition uniformity and wastes expensive materials. Rotating the substrate table is a technically inefficient method for achieving coating uniformity, because RF glow discharge plasma deposition is not a momentum transfer technique for the deposition of a thin film. Fixture rotation is common in sputtering and electron beam evaporation where a stream of particles is actually physically directed at the substrate to be coated. In RF glow discharge, such a technique is inconsistent with the physics of the reaction; if the plasma was uniform and the gas flows were uniform, substrate rotation would have no effects upon coating uniformity.
Sixth, both the teachings of Reinberg and the physical construction of the Applied Materials Plasma I reactor require that the substrates be heated to some several hundred degrees Centigrade. In practice, failure to heat the substrates according to the teachings of Reinberg and the instructions for use of the Applied Materials reactor causes totally unacceptable silicon nitride films (poor adhesion, porosity, very low deposition rates, etc.) However, heating silicon substrates is definitely not beneficial for the junctions and circuits resident thereon and in some cases is totally damaging thereto. Moreover, heating introduces stresses into the silicon nitride film when said films are cooled to room temperature.
U.S. Pat. No. 4,066,037 to Jacob describes an RF glow discharge apparatus as might be used to apply a series of films including silicon nitride. As with all plasma reactors, changing the composition of the reactant gases will change the chemistry of the film so produced. The teachings of Jacob are embodied in commercially available plasma deposition systems known as the System 8000 and System 8001 manufactured by LFE Corporation, Waltham, Mass. Jacobs teaches the use of a cylindrically shaped reaction chamber about which are wound a plurality of inductive coils. The coils are connected to an RF source which is used to excite an RF glow discharge plasma within said chamber having nitrogen gas flowing therethrough. As a result of the positive flow of the nitrogen gas, the plasma "tail flame" extends into the area where the substrate is mounted. The substrate is rigidly affixed to a mounting fixture held in a horizontal plane and connected to a heater so as to raise the temperature of the substrate during the deposition process. In addition, silane gas is piped into the region above the substrate through a plurality of orifices attempting a substantially even distribution of silane into the vicinity of the substrate. The nitrogen plasma is intended to energize the silane gas to the point of dissociation in order to permit the silicon and nitrogen species to form a silicon nitride film upon the surface of the substrate.
Jacob suffers from some of the same technical deficiencies as Reinberg heretofore discussed. First, the substrate is disposed in a horizontal attitude again permitting particulate contamination to fall upon the substrate surface and be included within the ultimately formed film. Second, the substrate must be heated thereby creating the same problem of internal stresses in films so produced and potential damage to circuits as discussed above. Third, the nitrogen plasma is required to perform the dissociation of the silane in order to form the silicon nitride film. However, it is unclear in Jacob exactly how the nitrogen species are to enter the space between the orifices supplying the silane and the substrate surface. The mixture of reactant gases is inherently uneven and subject to yielding uneven film quality and incorporating sufficient nitrogen therewithin. Fourth, as the substrate to be coated is not located within the area of excitation and formation of the RF glow discharge but downstream therefrom, nitrogen that has been dissociated to form nitrogen ions is permitted to significantly recombine by the time the nitrogen ions reach the area of the substrate. In order to minimize this effect, the flow rates of nitrogen gas are increased to an extent that physically urges the plasma into the area of the substrate. The high flow rates to achieve adequate excitation of the silane and an adequate presence of the nitrogen ions in the area of the substrate are inefficient, consume expensive reactant gases and produce a high volume of effluent "waste" gases that must be disposed of. Fifth, Jacob teaches that the substrate is rigidly mounted to a fixture at ground potential with respect to the RF excitation source. For reasons discussed heretofore with respect to Reinberg, it is undesirable to place the substrate at ground potential within a plasma deposition process because of the free electrons in such a region and the damage that can occur to the substrate surface during the very process which is meant to protect it (i.e., deposition of an impervious film of silicon nitride).
In addition to the prior art as described herein, a production plasma deposition system is manufactured by Tegal Corporation known as PLASMADEP 300. The PLASMADEP 300 is arranged in a rectangular configuration with reactant gases entering above from an inlet. Ammonia, silane, and nitrogen mix in an upper chamber ad pass through an etched metal screen which functions as the "hot" RF electrode for achieving the plasma. Substrates are mounted to a horizontal fixture which is at RF ground potential with heaters connected thereto. Operating at several hundred degrees centigrade, an RF plasma is created between the driven screen and the substrate fixture with substrates attached thereto. Waste gases exit from below the substrate fixture to a vacuum pumping system.
The PLASMADEP 300 suffers from some of the same inherent problems as discussed above for the other prior art plasma reactors. First, there is considerable direct flow of reactant gases around the substrate fixture that never enters the vicinity of the substrates for coating. Second, the flow of gases is substantially stagnated in the vicinity of some of the substrates causing an inherent flow variation of gases from sample to sample. Third, the inherent flow variation of gases causes sample coating nonuniformity. Fourth, the use of heat creates the same film stress problems and circuit boundary problems as discussed heretofore. Fifth, the efficiency of the reactor is dependent upon near complete mixing of the reactant gases. Otherwise, an uneven film will result. Sixth, the use of ammonia as a nitrogen donor to the silicon nitride film is corrosive and contaminating as discussed heretofore. Seventh, the use of a single plasma causes uneven dissociation of silane and nitrogen because of their widely divergent internal binding energies. Eighth, flow rates are extremely high in an attempt to achieve plasma uniformity and ultimate film uniformity through the use of an argon carrier gas, an expensive and inefficient solution to the inherently nonuniform gas flow of the reactor design.
For these reasons, the technology of the prior art has been inadequate to reliably create silicon nitride films by an RF glow discharge plasma which can withstand the rigors placed upon such a film by the semiconductor industry.